COUCHDB-2341 - do not remove id field for docs

After I finished the initial work I found a neat bug now that the custom `id` values are possible:

If we have a document with an **id** attribute `foo` and **_id**: `customid`
added from the user and we do a PUT request for an update,
CouchDB will return: {"ok":true, **id: "customid"**, rev: "123"}

The Model will think it has the id "mycustomid" instead of "foo"